Nonwoven fabric

ABSTRACT

Short fibers of thermoplastic synthetic resin are fusion bonded to one another to form a nonwoven fabric. The nonwoven fabric is formed on its upper surface with crests and troughs extending in parallel to one another. The crests include first crests having a uniform height dimension from a lower surface of the nonwoven fabric and second crests having a uniform height dimension from a lower surface of the nonwoven fabric which is smaller than the height dimension of the first crests. Density of the nonwoven fabric gradually increases in the order of the first crests, the second crests and the troughs. The first crests are formed so that the density of the first crests remains lower than the density of the second crests even when the first crests are compressed toward the lower surface until the first crests becomes flush with the level of the second crests.

TECHNICAL FIELD

The present invention relates to nonwoven fabrics formed of short fibersof thermoplastic synthetic resin.

It is conventionally known to fusion bond short fibers of thermoplasticsynthetic resin together and thereby to obtain nonwoven fabrics. As anexample of such nonwoven fabrics, JP 2008-25080 A (PTL 1) discloses anonwoven fabric having ridges and grooves extending in parallel to oneanother in a machine direction wherein these ridges and groovesalternate in a cross direction extending orthogonally to the machinedirection.

CITATION LIST Patent Literature

-   {PTL 1} JP 2008-25080 A

SUMMARY Technical Problem

The ridges in the nonwoven fabric disclosed in PTL 1 include therelatively high ridges and the relatively low ridges. The relativelyhigh ridges and the relatively low ridge are formed to be the same intheir basis mass wherein the basis mass of both these ridges havingdifferent height dimensions in the middle segments thereof in the widthdirection than the basis mass of the grooves. Assuming that such anonwoven fabric is used as the liquid-pervious inner sheet in thewearing article such as a disposable diaper or a menstruation napkin andcomes in tight contact with the wearer's skin, the relatively highridges are primarily compressed. In the inner sheet at this moment, thecompressed ridges have the density thereof further increased and, inconsequence, bodily fluids are apt to stay in these ridges and barredfrom smoothly moving toward the surrounding region of the lower density.When the ridges are compressed, for example, to the same level with therelatively low ridges, the ridges having been compressed in this mannerwill locally pressed against the wearer's skin. Eventually, the innersheet as a whole may not come in soft and close contact with thewearer's skin and may have an uncomfortable texture.

An object of this invention is to provide a nonwoven fabric improved sothat even when the nonwoven fabric is formed on its upper surface withthe ridges and the grooves, bodily fluids excreted onto the uppersurface may not stay in these ridges after this upper surface has comein close contact with the wearer's skin.

Solution to Problem

According to the present invention, there is provided a nonwoven fabricformed of short fibers of thermoplastic synthetic resin fusion bonded toone another and having a length direction, a width direction and athickness direction extending orthogonally one to another, including anupper surface and a lower surface opposed to the upper surface as viewedin the thickness direction wherein

the upper surface is formed with crests and troughs undulating in thewidth direction and extending in parallel to one another in the lengthdirection and

the crests include first crests having a uniform height dimension andsecond crests having a uniform height dimension measured from the lowersurface wherein the height dimension of the first crests is larger thanthe height dimension of the second crests.

The present invention resides in that a density of the nonwoven fabricgradually increases in order of the first crests, the second crests andthe troughs and the density of the first crests remains lower than thedensity of the second crests even when the first crests are compressedfrom the upper surface toward the lower surface until the first crestsbecome flush with the second crests.

According to one embodiment of the present invention, the nonwovenfabric has a basis mass in a range of 18 to 100 g/m² and each of theshort fibers has a fineness in a range of 1 to 8 dtex and a fiber lengthin a range of 20 to 80 mm, and the nonwoven fabric has been modified tobecome hydrophilic.

According to another embodiment of the present invention, the shortfibers are conjugate fibers including two types of the thermoplasticsynthetic resin having different fusion temperatures and these two typesof the thermoplastic synthetic resin are fusion bonded to each other viaone of the thermoplastic synthetic resins having a lower fusiontemperature.

According to still another embodiment of the present invention, theheight dimension of the first crests is in a range of 1 to 5 mm and theheight dimension of the second crests is lower than the height dimensionof the first crests by a range of 0.5 to 2 mm.

According to yet another embodiment of the present invention, the firstcrests and the second crests are formed alternately in the widthdirection and each of the troughs is interposed between each pair of theadjacent first crest and second crest.

A measuring method used for measuring “density of nonwoven fabric” willbe described later with reference to FIGS. 5 through 7.

Advantageous Effects of Invention

The nonwoven fabric according to the present invention has the crestsand the troughs extending in parallel to one another in the lengthdirection wherein the crests include the first crests each having auniform height dimension and the second crests each having a uniformheight dimension which is smaller than the height dimension of the firstcrests wherein both of the height dimensions are defined by a dimensionof the nonwoven fabric in the thickness direction and these crestscooperate with the troughs to form the upper surface of the nonwovenfabric with the ridges and the grooves. The density of the nonwovenfabric gradually increases in the order of the first crest, the secondcrest and the trough wherein the density of the first crest ismaintained lower than the density of the second crest even when thefirst crest is compressed to the same level as the second crest. On theassumption that such a nonwoven fabric is used as the inner sheet of thewearing article, even when the upper surface of the nonwoven fabriccomes in close contact with the wearer's skin and the first crestshaving initially been highest are compressed to the same level as thesecond crests, the density gradient of the first crests, the secondcrests and the troughs is maintained. In principle, bodily fluidsexcreted onto the inner sheet are apt to flow from the region having therelatively low density toward the region having the relatively highdensity, specifically, from the first crests to the second crests, thenfrom the second crest to the troughs. The stabilized density gradientassures that bodily fluids smoothly flow toward the troughs withoutstaying in the first crests and the wearer is free from discomfortfeeling of wetness even if the first crests directly come in contactwith the wearer's skin. In addition, the first crests would not locallypress against the wearer's skin since the density of the first crestsremains lower than that of the second crests. In other words, thenonwoven fabric according to the present invention has a uniformtexture.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial perspective view of a nonwoven fabric according tothe present invention.

FIG. 2 is a photograph of a section of the nonwoven fabric in a crossdirection.

FIG. 3 is a diagram partially illustrating a process for manufacturingthe nonwoven fabric.

FIG. 4 is a schematic front elevation view showing a first array of airjet nozzles.

FIG. 5 is a diagram illustrating how the section of the nonwoven fabricis transformed.

FIG. 6 is a graphic diagram plotting the surface contour of the nonwovenfabric based on observation.

FIG. 7 is a photograph showing a cross-section of fibers.

DESCRIPTION OF EMBODIMENTS

Details of nonwoven fabrics according to the present invention will bemore fully understood from the description given hereunder withreference to the accompanying drawings.

FIG. 1 is a partial perspective view of a nonwoven fabric 1 according tothe present invention and FIG. 2 is a 50-fold magnified photographexemplarily showing cross-section of the nonwoven fabric. It should benoted here that the photograph of FIG. 2 is really an assembly includinga plurality of photographs joined one with another in a width directionsince it is impossible to record the 50-fold magnified photograph of thenonwoven fabric over its sufficiently wide range in a single shot. Thenonwoven fabric 1 has a length direction, a width direction and athickness direction extending orthogonally one to another and designatedby the double-headed arrows A, B and C, respectively. The nonwovenfabric 1 has an upper surface 2 and a lower surface 3 opposed to eachother in the thickness direction C. The upper surface 2 is formed withcrests 6 and troughs 7 extending in parallel to one another in thelength direction A and undulating in the width direction B. The crests 6include first crests 6 a having a height H_(a) and second crests 6 bhaving a height H_(b) in the thickness direction C. The height H_(a) ofthe first crests 6 a is larger than the height H_(b) of the secondcrests 6 b. These first crests 6 a and second crests 6 b are arrangedalternately in the width dimension B and between each pair of theadjacent first and second crests 6 a, 6 b, each of the troughs 7 isinterposed.

Such nonwoven fabric 1 is formed by subjecting short fibers (staples) 11of thermoplastic synthetic resin preferably having a fineness in a rangeof 1 to 8 dtex and a fiber length in a range of 20 to 80 mm to blasts ofhot air and thereby fusion bonding them to one another. The nonwovenfabric 1 is suitable for use as a liquid-pervious inner sheet in abodily fluid-absorbent wearing article such as disposable diaper orsanitary napkin and, for such intended use, the nonwoven fabric 1preferably has a basis mass in a range of 18 to 100 g/m² and preferablyhas been previously treated to become hydrophilic. The nonwoven fabric 1used as the inner sheet includes a lower surface 3 being substantiallyflat and an upper surface 2 formed with troughs 7 defining grooves eachhaving a dimension W_(c) in the width direction B in a range of 0.4 to 2mm and first crests 6 a defining ridges each having a dimension W_(a) inthe width dimension B in a range of 2 to 5 mm, which is larger than adimension W_(b) of second crests 6 b in the width dimension B.Preferably, a height H_(a) of the first crests 6 a is in a range of 1 to5 mm, a height H_(b) of the second crests 6 b is lower than H_(a) by arange of 0.5 to 2 mm and a level of the troughs 7 is lower than theheight H_(a) by a range of 0.7 to 2.5 mm. To facilitate the short fibers11 to be fusion bonded to one another and to make the nonwoven fabric 1elastically compressible in the thickness direction C, each of the shortfibers 11 is preferably prepared in the form of a conjugate fiber madeof two types of synthetic resin having different fusion temperatures.Such conjugate fibers may be fusion bonded to one another by fusing thecomponent fibers of which fusion temperature is lower than the other.Combination of these different types of component synthetic resinincludes, for example, polyethylene/polyester orpolyethylene/polypropylene. The conjugate fiber may be of core-in-sheathtype and side-by-side type. The core-in-sheath type conjugate fiber maybe concentric core type or eccentric core type.

FIG. 3 is a diagram partially illustrating a process for manufacturingthe nonwoven fabric 1 and FIG. 4 is a schematic front elevation viewshowing a first array of air jet nozzles 910 used in the process of FIG.3. Referring to FIG. 3, a conveyor belt 200 which is air-permeable inthe thickness direction is loaded with a carded web 100 formed of theshort fibers 11 and runs in a machine direction MD. The carded web 100is subjected to blasts of a plurality of hot air jets 921 ejected from asecond air jet nozzle array 920 and the air jet 921 is sucked by asuction box 922 through the belt 200. The air jet 921 has a temperaturenot higher than a level to soften the short fibers 11 and serves tocompress the carded web 100 to a range of ⅔ to ¼ of its initialthickness and thereby to stabilize a texture of the carded web 100. Thenthe carded web 100 is subjected to blasts of a plurality of heated airjets 911 ejected from a first air jet nozzle array 910. The air jets 911has an airflow rate, a pressure and a temperature adjusted so that theshort fibers 11 may be moved in a cross direction CD orthogonal to themachine direction MD and may be fusion bonded to one another.

Referring to FIG. 4, the first air jet nozzle array 910 includes aplurality of nozzles (not shown) arranged at predetermined pitches a andb in the cross direction CD, from which the air jets 911 are ejectedtoward the carded web 100. These air jets 911 are sucked by a suctionbox 915 through the belt 200. The positions of the air jets 911 arrangedat the pitches a and b in the cross direction CD correspond to thepositions of the troughs 7 formed in the nonwoven fabric 1 as shown inFIG. 1. In the carded web 100, the short fibers 11 located just belowthe air jets 911 are partially shared half-and-half on both sides aboutthe cross direction CD, respectively, and these portions sharedhalf-and-half participate in formation of the first crests 6 a and thesecond crests 6 b while the portions staying immediately below the airjets 911 form the respective troughs 7. The short fibers 11 lyingbetween each pair of the adjacent air jets 911 are moved in the crossdirection CD to form the first crest 6 a or the second crest 6 b.Specifically, the short fibers 11 lying between each pair of theadjacent air jets 911 arranged at the relatively large pitch a move toform the first crest 6 a and the short fibers 11 lying between each pairof the adjacent air jets 911 arranged at the relatively small pitch bmove to form the second crest 6 b. The first crests 6 a formedconcurrently with the troughs 7 in this manner have a height larger thana height of the second crests 6 b and a density lower than a density ofthe second crests 6 b. In the troughs 7 formed immediately below therespective air jets 911, the short fibers 11 are compressed in thethickness direction C and densified.

When manufacturing the nonwoven fabric 1 by using the processillustrated in FIGS. 3 and 4, assuming that the carded web 100, forexample, having a basis mass in a range of 30 to 50 g/m² is conveyed inthe machine direction MD at a moderate speed, for example, at a speed inthe order of 10 m/min, an ejection quantity of the air jets 911 from thefirst air jet nozzle array 910 may be set to a low level and the secondair jet nozzle array 920 may be eliminated. On the assumption that suchcarded web 100 is conveyed at a speed in a range of 30 to 40 m/min, thesecond air jet nozzle array 920 may be used preferably in combinationwith the first air jet nozzle array 910 to stabilize the carded web 100in advance.

In the nonwoven fabric 1 of FIG. 1 obtained in this manner, a density ofthe first crests 6 a may be adjusted to be lower than a density of thesecond crests 6 b and a density of the second crests 6 b may be adjustedto be lower than a density of the troughs 7 to achieve a desirablebehavior of the nonwoven fabric 1 when such a nonwoven fabric is used asthe liquid-absorbent inner sheet of the bodily fluid-absorbent wearingarticle. Specifically, bodily fluids excreted on the upper surface 2 ofthe nonwoven fabric 1 move from the region having a low density towardthe region having a high density. In other words, bodily fluids smoothlymove from the first crests 6 a toward the troughs 7 and from the secondcrests 6 b toward the troughs 7 in accordance with the density gradientof the nonwoven fabric 1. When the nonwoven fabric 1 is bent and, inconsequence, the first crests 6 a and the second crests 6 b move closerto each other, bodily fluids move from the first crests 6 a toward thesecond crests 6 b. Therefore, certain amount of bodily fluids would notstay in the first crests 6 a being first to come in contact with thewearer's skin and would not create a discomfort feeling of wetnessagainst the wearer. In addition, even when the nonwoven fabric 1 iscompressed until the first crests 6 a are compressed to the same levelwith the second crests 6 b, the density of the first crests 6 a remainslower than the density of the second crests 6 b. Therefore, even in sucha situation, the first crests 6 a would not create a discomfort feelingof wetness against the wearer. Also, even when the first crests 6 a arecompressed together with the second crests 6 b, the first crests 6 asubjected to a higher degree of compression than that to which thesecond crests 6 b are subjected would not be pressed against thewearer's skin more tightly than the second crests 6 b, since the densityof the first crests 6 a is maintained lower than that of the secondcrests 6 b. Consequently, the texture of the nonwoven fabric would notbecome uneven and create a discomfort feeling to wear against thewearer.

TABLE 1 indicates primary manufacturing conditions and evaluationresults with respect to the nonwoven fabric 1 as one example of thepresent invention together with those with respect to comparativeexamples.

The nonwoven fabric according to the embodiment indicated in TABLE 1 wasmanufactured in the equipment exemplarily illustrated in FIGS. 3 and 4without using the second air jet nozzle array 920 under the primaryconditions as will be described below. In this embodiment, the runningspeed of the carded web 100 was sufficiently low to save use of thesecond air jet nozzle array 920.

(1) Short fiber (staple): core-in-sheath type conjugate fiber havingfineness in a range of 2.6 to 3.3 dtex, a fiber length in a range of 38to 51 mm, polyester as the core component and polyethylene as the sheathcomponent.(2) Carded web: basis mass of 35 g/m² and running speed of 10 m/min inthe machine direction.(3) First air jet nozzle array: nozzle diameter of 1 mm, nozzle pitcha=4 mm, nozzle pitch b=2.5 mm, distance between nozzle and conveyor beltof 5 mm, air jet pressure of 0.06 MPa and air temperature of 140° C.

The nonwoven fabric indicated in TABLE 1 as Comparative Example 1 wasobtained in the same manufacturing conditions as those for the Exampleaccording to the present invention except the nozzle pitches, i.e., setto a=4 mm and b=4 mm. The nonwoven fabric according to this ComparativeExample 1 is similar to the Example according to the present inventionin that the crests and the troughs arranged alternately in the crossdirection CD but the height of the crests is uniform and there is nodiscrimination between the first crests and the second crests.

The nonwoven fabric indicated in TABLE 1 as Comparative Example 2 wasobtained in the same manufacturing conditions as those for the Exampleaccording to the present invention except the nozzle pitches, i.e., setto a=3.5 mm and b=3 mm. The nonwoven fabric according to thisComparative Example 2 is similar to the Example according to the presentinvention in that the crests and the troughs arranged alternately in thecross direction CD and the crests include the first crests and thesecond crests having a smaller height than a height of the first crests.However, there is not a substantial difference in the height between thefirst crests and the second crests.

TABLE 1 Comparative Example Comparative Example 2 1st 2nd Example 1 1st2nd Crest Crest Trough Crest Trough Crest Crest Trough Non- Height (mm)2.43 1.45 — 2.07 — 1.91 1.53 — Compressed Width (mm) 3.81 2.08 0.56 3.520.44 2.82 2.17 0.62 state Sectional area (mm²) 5.71 1.78 0.16 4.03 0.133.31 2.13 0.17 Number of fiber's cross-sections 248.3 147.3 25.3 230.723 220 179.3 28 Fiber's density index 43.5 82.8 158.1 57.2 176.9 66.584.2 164.7 (number/mm²) Compressed Height (mm) 1.52 1.50 — 1.52 — 1.531.55 — state Width (mm) 3.80 1.99 0.56 3.48 0.46 2.77 2.19 0.64Sectional area (mm²) 5.07 1.73 0.16 3.59 0.16 1.77 2.10 0.15 Number offiber's cross-sections 248.3 147.3 25.3 230.7 23 220 179.3 28 Fiber'sdensity index 49.0 85.1 158.1 64.3 143.8 124.3 85.4 186.7 (number/mm²)Strike-through value (sec) 1.61 2.16 1.96 Q-Max value Load of 10 g/cm²0.178 0.373 0.366 (J/cm² · sec) Load of 30 g/cm² 0.819 1.027 0.863

(a) and (b) of FIG. 5 schematically illustrating cross-sectional shapeof the nonwoven fabric as the example of the present invention takenalong in the cross direction CD and (c) of FIG. 5 is a plan view of apressure plate 22 used in the step (b) of FIG. 5. In (a) of FIG. 5, thenonwoven fabric 1 is placed on a horizontal surface 21 (See FIG. 2 also)in a state free from any external force, i.e., in a non-compressed statewherein this nonwoven fabric 1 is formed with the first crests 6 a, thesecond crests 6 b and the troughs 7. In (b) of FIG. 5, the nonwovenfabric 1 was compressed by the pressure plate 22 from above until thefirst crests 6 a are deformed to be substantially flush with the secondcrests 6 b. The pressure plate 22 was supported by a pair of supporters25 having a height substantially the same as the height of the secondcrests 6 b. In the example of the present invention wherein the firstcrests 6 a have a height of 2.43 mm and the second crests 6 b have aheight of 1.45 mm, the nonwoven fabric was compressed until the heightof the first crests 6 a is reduced to 1.45 mm. The first crest 6 a, thesecond crest 6 b and the trough 7 respectively have widths W_(a), W_(b)and W_(c) and cross-sectional areas S_(a), S_(b) and S_(c) respectivelyindicated by shaded portions. A method to measure these cross-sectionalareas will be described later with reference to FIG. 6 and themeasurement result was indicated in TABLE 1.

In the nonwoven fabric as the comparative example 1, the nonwoven fabricin the non-compressed state was compressed by using the pressure plate22 shown in FIG. 5 until a height of the crests is reduced to 1.52 mm.Also in the nonwoven fabric as the comparative example 1,cross-sectional areas of the crest and the trough in the non-compressedstate as well as in the compressed state were measured and themeasurement result was indicated in TABLE 1.

In the nonwoven fabric as the comparative example 2, the nonwoven fabricin the non-compressed state was compressed by using the pressure plate22 until a height of the first crests which is relatively large in thenon-compressed state is reduced to a height of the second crests in thenon-compressed state, i.e., 1.52 mm. Also in the nonwoven fabric as thecomparative example 2, cross-sectional areas of the first crest and thesecond crest in the non-compressed state as well as in the compressedstate were measured and the measurement result was indicated in TABLE 1.

FIG. 6 is a graphic diagram plotting the cross-sectional areas S_(a),S_(b) and S_(c) of each of the first crests 6 a, the second crests 6 band the troughs 7 illustrated in FIG. 5, respectively, measured by 3Dmeasuring device wherein the first crest 6 a was partially illustrated.The nonwoven fabric 1 as an object to be measured has its lower surface3 placed on the horizontal surface 21 and undulation of the uppersurface as viewed in a cross-section taken along the cross direction CDwas recorded by an outline P. As 3D measuring device, High-AccuracyGeometry Measuring System (inclusive of High-Accuracy Stage: KS-1100)and High-Speed and High-Accuracy CCD-Laser Displacement Gauge inclusiveof Controller: LK-G3000V Set and Sensor Head: LK-G30) manufactured byKeyence Corporation were used and measuring conditions were set asfollows:

(Stage: KS-1100)

-   -   Range of measurement: 3000 μm×30000 μm    -   Measuring pitch: 20 μm    -   Running speed: 7500 μm/sec

(Measuring Head: LK-G3000V)

-   -   Measurement mode: Object to be measured    -   Installation mode: Diffuse reflection    -   Filtering: 4 times in average    -   Sampling period: 200 μm

The measurement record obtained by the 3D measuring device was processedby Configuration Analysis System KS-H1A (manufactured by KeyenceCorporation) to determine heights of the crests, widths of the crestsand the troughs and cross-sectional areas of the crests and the troughs.Referring to FIG. 6, respective widths W_(a), W_(b) and W_(c) of thefirst crest 6 a, the second crest 6 b and the trough 7 were determinedby a method as follows. Referring to FIG. 6, an intersection point X atwhich an arbitrary horizontal line H_(z) extending in parallel to thehorizontal surface 21 intersects with the outline P extending upwardfrom below the horizontal line H_(z) is obtained and, if a segment ofthe outline segment P extending between a pair of the adjacentintersecting points X lies below the horizontal line H_(z) and adistance between these two intersecting points X is in a range of 0.4 to2 mm, this segment extending between these intersecting points X wasdefined as the trough 7. A region extending between each pair of theadjacent troughs 7 is defined as the crest 6 and, of the adjacent twocrests 6, the crest 6 in which a top position of the outline P isrelatively high was defined as the first crest 6 a and the crest 6 inwhich a top position of the outline P is relatively low was defined asthe second crest 6 b. It should be noted here that each pair of theadjacent intersecting points X spaced from each other by a distance lessthan 0.4 mm was ignored and each pair of the adjacent intersectingpoints X spaced from each other by a distance of 0.4 mm or more wassearched. The distance by which each pair of the adjacent intersectingpoints X are spaced from each other corresponds to the width W_(c) ofthe trough 7. In the process for manufacturing the nonwoven fabric 1according to the present invention from the carded web 100, it is notpreferable for the present invention to adopt a trough having the widthsmaller than 0.4 mm because it will be difficult to form the first crest6 a and the second crest 6 b if the width W_(c) of the trough 7 isnarrower than 0.4 mm. Also when the width W_(c) exceeds 2 mm, it will bedifficult to manufacture the nonwoven fabric 1 according to the presentinvention and, if the nonwoven fabric including the troughs each havingsuch width W_(c) is used as the inner sheet of the bodilyfluid-absorbent article, the distance between each pair of the adjacentcrests 6 will be too large to assure a desired texture. For this reason,it is not preferable to adopt such excessively wide troughs 7.

With respect also to the nonwoven fabric 1 compressed by the pressureplate 22 until the first crests 6 a are compressed downward to the stateas illustrated in (b) of FIG. 5, the outline P was plotted by theprocedure illustrated in FIG. 6 and thereby the heights H_(a), H_(b),the widths W_(a), W_(b), W_(c) and the cross-sectional areas S_(a),S_(b) and S_(c) were measured. Referring to (c) of FIG. 5, the pressureplate 22 is formed in its middle region as viewed in the width directionwith a slit 26 extending through the pressure plate 22 in its thicknessdirection. This slit 26 allows the 3D measuring device to be operated sothat the sensor head of the laser displacement gauge in the 3D measuringdevice may observe the crests 6 and the troughs 7 of the nonwoven fabric1 in the course of measuring the heights H_(a), H_(b) and the widthW_(a), W_(b), W_(c) in the nonwoven fabric 1 in the compressed state.The result obtained from the measurement having been carried out in thismanner is recorded in TABLE 1. Measurement of the height, the width andthe cross-sectional area was carried out also on the nonwoven fabric ascomparative examples 1 and 2 in the same manner as in the nonwovenfabric 1 according to the present invention. The result of thesemeasurements is also recorded in TABLE 1.

FIG. 7 is a 50-fold magnified photograph exemplarily showing across-section of the second crest 6 b cut in the cross direction CD. Thecut surface was obtained by cutting the nonwoven fabric 1 in the crossdirection CD using a sharp cutter, for example, the substitute edgeH-100 for KOKUYO Cutter Knife HA-B (trade name). This cut surface wasphotographed at 50-fold magnifications by using Real SurfaceView-VE-7800 manufactured by Keyence Corporation. Cross-sections 11 a ofthe short fibers 11 appear in this photograph of the cross-sectionobtained in this manner. According to the present invention, the numberof the cross-sections 11 a (i.e., the number of fiber cross-sections)was determined with respect to the first crests 6 a, the second crests 6b and the troughs 7, then respective ratios between these numbers andthe cross-sectional areas of the first crests 6 a, the second crests 6 band the troughs 7 were determined and the respective values of theseratios were defined as the respective fiber densities of the firstcrests 6 a, the second crests 6 b and the troughs 7 in the compressedstate and in the non-compressed state. In this invention, these fiberdensities are referred to also as fiber density indices. TABLE 1indicates the fiber density indices in the example of the presentinvention and the comparative examples 1 and 2. As will be apparent fromTABLE 1, the fiber density indices of the nonwoven fabric in the exampleof the present invention, the fiber density indices of the nonwovenfabric in the non-compressed state gradually increased in the order ofthe first crests 6 a, the second crests 6 b and the troughs 7. Suchgradient of the fiber density indices was not changed in the compressedstate also. Specifically, in the nonwoven fabric according to thepresent invention, even when the first crests 6 a are compressed to thesame level as the second crests 6 b, the fiber density of the firstcrests 6 a remains lower than that of the second crests 6 b andtherefore bodily fluids excreted on the upper surface 2 of the nonwovenfabric 1 may smoothly move from the first crests 6 a having a relativelylow density to the second crests 6 b and the troughs 7 having arelatively high density.

When such a nonwoven fabric 1 is used as an inner sheet of an absorbentarticle of such as, for example, a disposable diaper, the nonwovenfabric 1 allows bodily fluids to move quickly from the upper surface 2toward the lower surface 3 and this capacity can be represented, forexample, by the strike-through value or Q-Max value.

The strike-through value used herein is represented by a time (unit:sec) required for 10 ml of artificial urine to pass through the nonwovenfabric in the form of a test piece for measurement and the smaller thestrike-through value, the sooner the permeation. To measure thestrike-through value, the tester LISTER manufactured by Lenzing TechnikCorporation was used. Specifically, the measuring probe was placed onthe nonwoven fabric and the tester was operated in accordance withEDANA-ERT Section 150.3 liquid strike-though time method prescribed forthis tester. Under the nonwoven fabric, 20 sheets of filter paper(Qualitative filter paper No. 2 manufactured by ADVANTEC MFS., INC.)were stacked in substitution for the absorbent article. The artificialurine was prepared by dissolving 200 g of urea, 80 g of sodium chloride,8 g of magnesium sulfate, 3 g of calcium chloride and 1 g of bluepigment No. 1 in 10 liter of ion-exchange water and 72 mN/m of thisartificial urine was used at a temperature of 20° C. Result ofmeasurement was indicated in TABLE 1. The strike-through values measuredon the comparative examples 1 and 2 were also indicated in TABLE 1.

The Q-Max value corresponds to a quantified heat quantity drawn from thewearer's skin by the inner sheet when the inner sheet wetted with bodilyfluids comes in contact with the wearer's skin represented by unit ofJ/cm²*sec. The higher the Q-Max value of the inner sheet is, the largerthe heat quantity drawn from the wearer's skin becomes and, inconsequence, the wearer experiences an abrupt cold sensation. To measurethe Q-Max value, KES-F7-THERMOLABO II Model high-accuracy and high-speedthermal property measuring device manufactured by KATO TECH CO., LTD.was used. As the measuring conditions, a temperature of the probe wasset to <room temperature+10° C., Q-Max measurement (cool sensitivitymeasurement)> and as the surface temperature of the probe, two standardlevels of 10 g/cm² and 30 g/cm² were adopted. As the nonwoven fabric formeasurement, 10×10 cm was used and this nonwoven fabric was placed on 3sheets of qualitative filter paper No. 2 stacked, a cylinder having aninner diameter of 20 mm was placed on the nonwoven fabric and 10 cc ofthe artificial urine was poured into this cylinder about 5 sec, then thecylinder was removed, 20 sec after the artificial urine had been poured,the probe was put in contact with the surface of the nonwoven fabric tomeasure a heat transfer from the probe to the surface of the nonwovenfabric. Result of measurement was indicated in TABLE 1. As will beapparent from comparison of the example of the invention with thecomparative examples, the nonwoven fabric 1 free from possibility thatbodily fluids might stay on the surface 2 exhibited a relatively smallQ-Max value.

REFERENCE SIGNS LIST

-   1 nonwoven fabric-   2 upper surface-   3 lower surface-   6 crest-   6 a crest (first crest)-   6 b crest (second crest)-   7 troughs-   11 short fibers-   A length direction-   B width direction-   C thickness direction

1. A nonwoven fabric formed of short fibers of thermoplastic syntheticresin fusion bonded to one another and having a length direction, awidth direction and a thickness direction extending orthogonally to oneanother, including an upper surface and a lower surface opposed to theupper surface as viewed in the thickness direction wherein the uppersurface is formed with crests and troughs undulating in the widthdirection and extending in parallel to one another in the lengthdirection and the crests comprise first crests having a uniform heightdimension and second crests having a uniform height dimension measuredfrom the lower surface wherein the height dimension of the first crestsis larger than the height dimension of the second crests, wherein:density of the nonwoven fabric gradually increases in order of the firstcrests, the second crests and the troughs and the density of the firstcrests remains lower than the density of the second crests even when thefirst crests are compressed from the upper surface toward the lowersurface until the first crests become flush with the second crests. 2.The nonwoven fabric defined by claim 1, wherein the nonwoven fabric hasa basis mass in a range of 18 to 100 g/m² and each of the short fibershas a fineness in a range of 1 to 8 dtex and a fiber length in a rangeof 20 to 80 mm, and wherein the nonwoven fabric has been modified tobecome hydrophilic.
 3. The nonwoven fabric defined by claim 1, whereinthe short fibers are conjugate fibers comprising two types of thethermoplastic synthetic resin having different fusion temperatures andthese two types of the thermoplastic synthetic resin are fusion bondedto each other via one of the thermoplastic synthetic resins having alower fusion temperature.
 4. The nonwoven fabric defined by claim 1,wherein the height dimension of the first crests is in a range of 1 to 5mm and the height dimension of the second crests is lower than theheight dimension of the first crests by a range of 0.5 to 2 mm.
 5. Thenonwoven fabric defined by claim 1, wherein the first crests and thesecond crests are formed alternately in the width direction and each ofthe troughs is interposed between each pair of the adjacent the firstcrest and the second crest.